As the frequency of operation of a magnetic component such as a transformer increases, the depth to which current penetrates the conductors comprising the transformer decreases. This penetration depth is referred to as "skin depth". At room temperature, copper has a skin depth .delta., measured in mils, equal to 2.60.sqroot.1/f, where f represents frequency in Hertz. Thus, at a frequency of about 1 megahertz, current penetration in copper is only on the order of 2.6 mils. Consequently, if the conductors are more than several skin depths thick, then any portion of the conductors which is farther than 3 skin depths or 3.delta. from the exterior surface is not involved in carrying the transformer currents. For high frequency operation, inactive conductive material adds to the weight and volume of the transformer without enhancing its operational characteristics. Therefore, it is desirable to make magnetic components, such as transformers, as small as possible. To this end, planar conductive films are used which have a thickness on the order of twice the skin depth at the intended operating frequency of the magnetic component's conductors. These conductive films are normally disposed on a dielectric membrane and patterned to provide the desired winding configuration. Multiturn windings normally comprise either a single layer spiral or a stack of layers of individual conductive films on dielectric substrates which are interconnected layer-to-layer with soldered connecting bars to provide a continuous winding. Spiral windings are limited in the number of turns they can provide for high currents, and multilayer windings have the disadvantage of requiring a number of layer-to-layer connections which increases with the number of turns in the winding. The process of connecting thin conductive layers in layer-to-layer fashion with connecting bars, which are soldered to the edge of the conductor, is exacting and tends to have a poor yield since the solder can easily short-circuit layers or fail to connect a layer or layers, either situation resulting in an inoperative winding.
Copending U.S. patent application Ser. No. 359,063 of K. D. T. Ngo and A. J. Yerman, filed on May 30, 1989 and assigned to the instant assignee, now U.S. Pat. No. 5,017,902, which is hereby incorporated by reference, describes a conductive film magnetic component having a pattern which, when folded in an accordion-like manner, produces a multiturn winding for which layer-to-layer connections are built into the pattern of the conductive film and the folding thereof. Dielectric material spaces apart adjacent layers of the folded conductive film in order to prevent unintended connections between the different layers. The winding of the cited patent application may comprise either an inductor or the primary winding of a transformer. For a transformer, a secondary winding is provided by patterning a conductive film and a dielectric substrate such that folding the dielectric substrate results in a multilayer stack of two-turn, centertapped windings. These center-tapped windings are connected in parallel to form the secondary winding which is interleaved with the hereinabove described primary winding to form a transformer. Such a transformer structure is less complex to manufacture and assemble than prior art conductive film transformers and is more reliable.
As a further improvement over the prior art, it is desirable to provide a thin film, high-frequency transformer for which the primary winding and one or more secondary windings are constructed as part of the same flexible circuit comprising conductive films and dielectric material, the pattern of which, when folded, provides a complete and compact transformer assembly.